1. Field of the Invention
This invention relates to a process for the preparation of chloromethyltetrachlorobenzenes and particularly to the preparation of methylchloromethyltetrachlorobenzenes and/or di-(chloromethyl)-tetrachlorobenzenes by the chlorination of xylenes. This invention is particularly directed to a highly selective process for the preparation of said chloromethyltetrachlorobenzenes whereby photochlorination is effected by directing the chlorine gas into a vapor space above a xylene solution and agitating the solution while directing a source of high energy light at such vapor space. This invention is also directed to such a process whereby high yields of the chloromethyltetrachlorobenzenes are recovered at high purities.
2. Discussion of the Prior Art
According to the U.S. Pat. No. 2,412,389, it is known to chlorinate p-xylene in the presence of FeCl.sub.3 in CCl.sub.4 to prepare 1,4-dimethyl-2,3,5,6-tetrachlorobenzene, and after the removal of the FeCl.sub.3, to chlorinate the latter with irradiation and refluxing in CCl.sub.4 to form 1,4-di-(chloromethyl)-2,3,5,6-tetrachlorobenzene. According to Belgian Pat. No. 631,170, it is likewise known first to prepare 1,3-dimethyl-2,4,5,6-tetrachlorobenzene from m-xylene, and 1,2-dimethyl-3,4,5,6-tetrachlorobenzene from o-xylene, and to prepare from these, by photochlorination, 1,3-di-(chloromethyl)-2,4,5,6-tetrachlorobenzene and 1,2-di-(chloromethyl)-3,4,5,6-tetrachlorobenzene, respectively.
German Pat. No. 1,568,607 describes a method of chlorinating xylenes in the presence of FeCl.sub.3 to form tetrachloroxylenes, and then, in a second reaction, without removal of the FeCl.sub.3, performing further chlorination with irradiation with high-energy light to form di-(chloromethyl)-tetrachlorobenzenes.
In comparison with the process of U.S. Pat. No. 2,412,389 and Belgian Pat. No. 631,170, in which the xylene chlorinated in the nucleus is isolated, refined, and then photochlorinated to di-(chloromethyl)-tetrachlorobenzenes, the process of German Pat. No. 1,568,607 appears to represent an advance in that the xylene chlorinated in the nucleus is not isolated, but is photochlorinated in the form of a suspension in CCl.sub.4 containing ferric chloride to produce di-(chloromethyl)-tetrachlorobenzene. The photochlorination requires that the ferric chloride concentration be as low as possible. The German Pat. No. 1,568,607 recommends 0.05 to 1.0 weight percent FeCl.sub.3 with respect to xylene, in order to minimize the absorption of the light required for the activation of chlorine.
However, due to the fact that the side-chain chlorination must take place in the presence of low amounts of ferric chloride the reaction requires operations with a high chlorine input rate (0.4 to 1.6 parts by weight per weight-part of xylene per hour). This high chlorine input makes up for the decrease in the output of catalyst. In an example given for purposes of comparison in German Pat. No. 1,568,607 it is shown what a disadvantageous effect a slower introduction of chlorine has on the yield of dimethyltetrachlorobenzene under otherwise the same conditions. On the other hand, it is generally known that the slower the introduction of chlorine is under otherwise the same conditions, the more fully it is utilized.
The chlorine utilization in the nuclear chlorination of xylene pursuant to German Pat. No. 1,568,607 is accordingly poor. 140% of the stoichiometric amount of chlorine is required.
In the side-chain photochlorination to di-(chloromethyl)-tetrachlorobenzene of German Pat. No. 1,568,607, chlorine is introduced (evidently for better chlorine utilization) at a rate that is one-third to two-thirds lower than the rate required for the chlorination in the nucleus, with heating and irradiation from a submerged ultraviolet lamp of 80 W.
According to German Pat. No. 1,568,607, this procedure is said to produce 1,4-di-(chloromethyl)-tetrachlorobenzene in a 98.5% yield, but efforts to confirm this have proven unsuccessful. Although the conditions specified for the chlorination in the nucleus were accurately created as specified in the patent, and the side-chain chlorination was performed at half of the chlorine introduction rate, even using 200% of the calculated stoichiometric amount of chlorine (German Pat. No. 1,568,607 recommends one-third to two-thirds of the nuclear chlorination rate for the side-chain chlorination, which in Example 2, at 5 hours of chlorine feeding, amounts to 133 to 266% of the calculated amounted of chlorine), only 10.6% 1,4-dimethyltetrachlorobenzene, 74.4% 1-methyl-4-chloromethyltetrachlorobenzene, and 13.7% 1,4-di-(chloromethyl)-tetrachlorobenzene were obtained.
Apparently, if the teaching of German Pat. No. 1,568,607 is followed, the transformation and the reaction rate of the side-chain photochlorination are low on account of the absorption of the high-energy light by the ferric chloride present in the solution during the photochlorination.
It has therefore become desirable to provide a process for the preparation of chloromethyltetrachlorobenzenes whereby the intermediate product obtained from the nuclear chlorination need not be recovered and refined prior to photochlorination of the side-chain. It has become desirable, however, to provide a process which can be conducted in a sequential manner whereby the amount of chlorine employed is minimized. It has also become desirable to provide a process which is selective in respect of the desired chloromethyltetrachlorobenzene. It has become particularly desirable to provide a process in which the amount of chlorine employed will not exceed about 130% of the stoichiometric amount and yet provide yields of the desired chloromethyltetrachlorobenzene in excess of 60%.